123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159160161162163164165166167168169170171172173174175176177178179180181182183184185186187188189190191192193194195196197198199200201202203204205206207208209210211212213214215216217218219220221222223224225226227228229230231232233234235236237238239240241242243244245246247248249250251252253254255256257258259260261262263264265266267268269270271272273274275276277278279280281282283284285286287288289290291292293294295296297298299300301302303304305306307308309310311312313314315316317318319320321322323324325326327328329330331332333334335336337338339340341342343344345346347348349350351352353354355356357358359360361362363364365366367368369370371372373374375376377378379380381382383384385386387388389390391392393394395396397398399400401402403404405406407408409410411412413414415416417418419420421422423424425426427428429430431432433434435436437438439440441442443444445446447448449450451452453454455456457458459460461462463464465466467468469470471472473474475476477478479480481482483484485486487488489490491492493494495496497498499500501502503504505506507508509510511512513514515516517518519520521522523524525526527528529530531532533534535536537538539540541542543544545546547548549550551552553554555556557558559560561562563564565566567568569570571572573574575576577578579580581582583584585586587588589590591592593594595596597598599600601602603604605606607608609610611612613614615616617618619620621622623624625626627628629630631632633634635636637638639640641642643644645646647648649650651652653654655656657658659660661662663664665666667668669670671672673674675676677678679680681682683684685686687688689690691692693694695696697698699700701702703704705706707708709710711712713714715716717718719720721722723724725726727728729730731732733734735736737738739740741742743744745746747748749750751752753754755756757758759760761762763764765766767768769770771772773774775776777778779780781782783784785786787788789790791792793794795796797798799800801802803804805806807808809810811812813814815816817818819820821822823824825826827828829830831832833834835836837838839840841842843844845846847848849850851852853854855856857858859860861862863864865866867868869870871872873874875876877878879880881882883884885886887888889890891892893894895896897898899900901902903904905906907908909910911912913914915916917918919920921922923924925926927928929930931932933934935936937938939940941942943944945946947948949950951952953954955956957958959960961962963964965966967968969970971972973974975976977978979980981982983984985986987988989990991992993994995996997998999100010011002100310041005100610071008100910101011101210131014101510161017101810191020102110221023102410251026102710281029103010311032103310341035103610371038103910401041104210431044104510461047104810491050105110521053105410551056105710581059106010611062106310641065106610671068106910701071107210731074107510761077107810791080108110821083108410851086108710881089109010911092109310941095109610971098109911001101110211031104110511061107110811091110111111121113111411151116111711181119112011211122112311241125112611271128112911301131113211331134113511361137113811391140114111421143114411451146114711481149115011511152115311541155115611571158115911601161116211631164116511661167116811691170117111721173117411751176117711781179118011811182118311841185118611871188118911901191119211931194119511961197119811991200120112021203120412051206120712081209121012111212121312141215121612171218121912201221122212231224122512261227122812291230123112321233123412351236123712381239124012411242124312441245124612471248124912501251125212531254125512561257125812591260126112621263126412651266126712681269127012711272127312741275127612771278127912801281128212831284128512861287128812891290129112921293129412951296129712981299130013011302130313041305130613071308130913101311131213131314131513161317131813191320132113221323132413251326132713281329133013311332133313341335 |
- <!DOCTYPE HTML>
- <html lang="zh-hans" >
- <head>
- <meta charset="UTF-8">
- <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
- <title>4-10 使用 Tree Shaking · 深入浅出 Webpack</title>
- <meta http-equiv="X-UA-Compatible" content="IE=edge" />
- <meta name="description" content="">
- <meta name="generator" content="GitBook 3.2.3">
- <meta name="author" content="吴浩麟">
- <meta name="identifier" content="9787121331725" scheme="ISBN">
-
-
- <link rel="stylesheet" href="../gitbook/style.css">
-
-
-
- <link rel="stylesheet" href="../gitbook/gitbook-plugin-modal/index.css">
-
-
-
- <link rel="stylesheet" href="../gitbook/gitbook-plugin-highlight/website.css">
-
-
-
- <link rel="stylesheet" href="../gitbook/gitbook-plugin-search/search.css">
-
-
-
- <link rel="stylesheet" href="../gitbook/gitbook-plugin-fontsettings/website.css">
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
- <meta name="HandheldFriendly" content="true"/>
- <meta name="viewport" content="width=device-width, initial-scale=1, user-scalable=no">
- <meta name="apple-mobile-web-app-capable" content="yes">
- <meta name="apple-mobile-web-app-status-bar-style" content="black">
- <link rel="apple-touch-icon-precomposed" sizes="152x152" href="../gitbook/images/apple-touch-icon-precomposed-152.png">
- <link rel="shortcut icon" href="../gitbook/images/favicon.ico" type="image/x-icon">
-
- <link rel="next" href="4-11提取公共代码.html" />
-
-
- <link rel="prev" href="4-9CDN加速.html" />
-
- <style>
- @media only screen and (max-width: 640px) {
- .book-header .hidden-mobile {
- display: none;
- }
- }
- </style>
- <script>
- window["gitbook-plugin-github-buttons"] = {"buttons":[{"user":"gwuhaolin","type":"follow","size":"small","width":"150"},{"user":"gwuhaolin","repo":"dive-into-webpack","type":"star","size":"small"}]};
- </script>
- </head>
- <body>
-
- <div class="book">
- <div class="book-summary">
-
-
- <div id="book-search-input" role="search">
- <input type="text" placeholder="输入并搜索" />
- </div>
-
- <nav role="navigation">
-
- <ul class="summary">
-
-
-
-
-
-
- <li class="chapter " data-level="1.1" data-path="../">
-
- <a href="../">
-
-
- 深入浅出 Webpack
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.2" data-path="../前言.html">
-
- <a href="../前言.html">
-
-
- 前言
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.3" data-path="../1入门/">
-
- <a href="../1入门/">
-
-
- 第1章 入门
-
- </a>
-
-
- <ul class="articles">
-
- <li class="chapter " data-level="1.3.1" data-path="../1入门/1-1前端的发展.html">
-
- <a href="../1入门/1-1前端的发展.html">
-
-
- 1-1 前端的发展
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.3.2" data-path="../1入门/1-2常见的构建工具及对比.html">
-
- <a href="../1入门/1-2常见的构建工具及对比.html">
-
-
- 1-2 常见的构建工具及对比
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.3.3" data-path="../1入门/1-3安装与使用.html">
-
- <a href="../1入门/1-3安装与使用.html">
-
-
- 1-3 安装与使用
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.3.4" data-path="../1入门/1-4使用Loader.html">
-
- <a href="../1入门/1-4使用Loader.html">
-
-
- 1-4 使用 Loader
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.3.5" data-path="../1入门/1-5使用Plugin.html">
-
- <a href="../1入门/1-5使用Plugin.html">
-
-
- 1-5 使用 Plugin
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.3.6" data-path="../1入门/1-6使用DevServer.html">
-
- <a href="../1入门/1-6使用DevServer.html">
-
-
- 1-6 使用 DevServer
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.3.7" data-path="../1入门/1-7核心概念.html">
-
- <a href="../1入门/1-7核心概念.html">
-
-
- 1-7 核心概念
-
- </a>
-
-
- </li>
- </ul>
-
- </li>
- <li class="chapter " data-level="1.4" data-path="../2配置/">
-
- <a href="../2配置/">
-
-
- 第2章 配置
-
- </a>
-
-
- <ul class="articles">
-
- <li class="chapter " data-level="1.4.1" data-path="../2配置/2-1Entry.html">
-
- <a href="../2配置/2-1Entry.html">
-
-
- 2-1 Entry
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.4.2" data-path="../2配置/2-2Output.html">
-
- <a href="../2配置/2-2Output.html">
-
-
- 2-2 Output
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.4.3" data-path="../2配置/2-3Module.html">
-
- <a href="../2配置/2-3Module.html">
-
-
- 2-3 Module
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.4.4" data-path="../2配置/2-4Resolve.html">
-
- <a href="../2配置/2-4Resolve.html">
-
-
- 2-4 Resolve
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.4.5" data-path="../2配置/2-5Plugins.html">
-
- <a href="../2配置/2-5Plugins.html">
-
-
- 2-5 Plugins
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.4.6" data-path="../2配置/2-6DevServer.html">
-
- <a href="../2配置/2-6DevServer.html">
-
-
- 2-6 DevServer
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.4.7" data-path="../2配置/2-7其它配置项.html">
-
- <a href="../2配置/2-7其它配置项.html">
-
-
- 2-7 其它配置项
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.4.8" data-path="../2配置/2-8整体配置结构.html">
-
- <a href="../2配置/2-8整体配置结构.html">
-
-
- 2-8 整体配置结构
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.4.9" data-path="../2配置/2-9多种配置类型.html">
-
- <a href="../2配置/2-9多种配置类型.html">
-
-
- 2-9 多种配置类型
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.4.10" data-path="../2配置/2-10配置总结.html">
-
- <a href="../2配置/2-10配置总结.html">
-
-
- 2-10 配置总结
-
- </a>
-
-
- </li>
- </ul>
-
- </li>
- <li class="chapter " data-level="1.5" data-path="../3实战/">
-
- <a href="../3实战/">
-
-
- 第3章 实战
-
- </a>
-
-
- <ul class="articles">
-
- <li class="chapter " data-level="1.5.1" data-path="../3实战/3-1使用ES6语言.html">
-
- <a href="../3实战/3-1使用ES6语言.html">
-
-
- 3-1 使用 ES6 语言
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.5.2" data-path="../3实战/3-2使用TypeScript语言.html">
-
- <a href="../3实战/3-2使用TypeScript语言.html">
-
-
- 3-2 使用 TypeScript 语言
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.5.3" data-path="../3实战/3-3使用Flow检查器.html">
-
- <a href="../3实战/3-3使用Flow检查器.html">
-
-
- 3-3 使用 Flow 检查器
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.5.4" data-path="../3实战/3-4使用SCSS语言.html">
-
- <a href="../3实战/3-4使用SCSS语言.html">
-
-
- 3-4 使用 SCSS
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.5.5" data-path="../3实战/3-5使用PostCSS.html">
-
- <a href="../3实战/3-5使用PostCSS.html">
-
-
- 3-5 使用 PostCSS
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.5.6" data-path="../3实战/3-6使用React框架.html">
-
- <a href="../3实战/3-6使用React框架.html">
-
-
- 3-6 使用 React 框架
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.5.7" data-path="../3实战/3-7使用Vue框架.html">
-
- <a href="../3实战/3-7使用Vue框架.html">
-
-
- 3-7 使用 Vue 框架
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.5.8" data-path="../3实战/3-8使用Angular2框架.html">
-
- <a href="../3实战/3-8使用Angular2框架.html">
-
-
- 3-8 使用 Angular2 框架
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.5.9" data-path="../3实战/3-9为单页应用生成HTML.html">
-
- <a href="../3实战/3-9为单页应用生成HTML.html">
-
-
- 3-9 为单页应用生成 HTML
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.5.10" data-path="../3实战/3-10管理多个单页应用.html">
-
- <a href="../3实战/3-10管理多个单页应用.html">
-
-
- 3-10 管理多个单页应用
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.5.11" data-path="../3实战/3-11构建同构应用.html">
-
- <a href="../3实战/3-11构建同构应用.html">
-
-
- 3-11 构建同构应用
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.5.12" data-path="../3实战/3-12构建Electron应用.html">
-
- <a href="../3实战/3-12构建Electron应用.html">
-
-
- 3-12 构建 Electron 应用
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.5.13" data-path="../3实战/3-13构建Npm模块.html">
-
- <a href="../3实战/3-13构建Npm模块.html">
-
-
- 3-13 构建 Npm 模块
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.5.14" data-path="../3实战/3-14构建离线应用.html">
-
- <a href="../3实战/3-14构建离线应用.html">
-
-
- 3-14 构建离线应用
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.5.15" data-path="../3实战/3-15搭配NpmScript.html">
-
- <a href="../3实战/3-15搭配NpmScript.html">
-
-
- 3-15 搭配 Npm Script
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.5.16" data-path="../3实战/3-16检查代码.html">
-
- <a href="../3实战/3-16检查代码.html">
-
-
- 3-16 检查代码
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.5.17" data-path="../3实战/3-17通过Node.jsAPI启动Webpack.html">
-
- <a href="../3实战/3-17通过Node.jsAPI启动Webpack.html">
-
-
- 3-17 通过 Node.js API 启动 Webpack
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.5.18" data-path="../3实战/3-18使用WebpackDevMiddleware.html">
-
- <a href="../3实战/3-18使用WebpackDevMiddleware.html">
-
-
- 3-18 使用 Webpack Dev Middleware
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.5.19" data-path="../3实战/3-19加载图片.html">
-
- <a href="../3实战/3-19加载图片.html">
-
-
- 3-19 加载图片
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.5.20" data-path="../3实战/3-20加载SVG.html">
-
- <a href="../3实战/3-20加载SVG.html">
-
-
- 3-20 加载SVG
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.5.21" data-path="../3实战/3-21加载SourceMap.html">
-
- <a href="../3实战/3-21加载SourceMap.html">
-
-
- 3-21 加载 Source Map
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.5.22" data-path="../3实战/3-22实战总结.html">
-
- <a href="../3实战/3-22实战总结.html">
-
-
- 3-22 实战总结
-
- </a>
-
-
- </li>
- </ul>
-
- </li>
- <li class="chapter " data-level="1.6" data-path="./">
-
- <a href="./">
-
-
- 第4章 优化
-
- </a>
-
-
- <ul class="articles">
-
- <li class="chapter " data-level="1.6.1" data-path="4-1缩小文件搜索范围.html">
-
- <a href="4-1缩小文件搜索范围.html">
-
-
- 4-1 缩小文件搜索范围
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.6.2" data-path="4-2使用DllPlugin.html">
-
- <a href="4-2使用DllPlugin.html">
-
-
- 4-2 使用 DllPlugin
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.6.3" data-path="4-3使用HappyPack.html">
-
- <a href="4-3使用HappyPack.html">
-
-
- 4-3 使用 HappyPack
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.6.4" data-path="4-4使用ParallelUglifyPlugin.html">
-
- <a href="4-4使用ParallelUglifyPlugin.html">
-
-
- 4-4 使用 ParallelUglifyPlugin
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.6.5" data-path="4-5使用自动刷新.html">
-
- <a href="4-5使用自动刷新.html">
-
-
- 4-5 使用自动刷新
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.6.6" data-path="4-6开启模块热替换.html">
-
- <a href="4-6开启模块热替换.html">
-
-
- 4-6 开启模块热替换
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.6.7" data-path="4-7区分环境.html">
-
- <a href="4-7区分环境.html">
-
-
- 4-7 区分环境
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.6.8" data-path="4-8压缩代码.html">
-
- <a href="4-8压缩代码.html">
-
-
- 4-8 压缩代码
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.6.9" data-path="4-9CDN加速.html">
-
- <a href="4-9CDN加速.html">
-
-
- 4-9 CDN 加速
-
- </a>
-
-
- </li>
- <li class="chapter active" data-level="1.6.10" data-path="4-10使用TreeShaking.html">
-
- <a href="4-10使用TreeShaking.html">
-
-
- 4-10 使用 Tree Shaking
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.6.11" data-path="4-11提取公共代码.html">
-
- <a href="4-11提取公共代码.html">
-
-
- 4-11 提取公共代码
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.6.12" data-path="4-12按需加载.html">
-
- <a href="4-12按需加载.html">
-
-
- 4-12 按需加载
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.6.13" data-path="4-13使用Prepack.html">
-
- <a href="4-13使用Prepack.html">
-
-
- 4-13 使用 Prepack
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.6.14" data-path="4-14开启ScopeHoisting.html">
-
- <a href="4-14开启ScopeHoisting.html">
-
-
- 4-14 开启 Scope Hoisting
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.6.15" data-path="4-15输出分析.html">
-
- <a href="4-15输出分析.html">
-
-
- 4-15 输出分析
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.6.16" data-path="4-16优化总结.html">
-
- <a href="4-16优化总结.html">
-
-
- 4-16 优化总结
-
- </a>
-
-
- </li>
- </ul>
-
- </li>
- <li class="chapter " data-level="1.7" data-path="../5原理/">
-
- <a href="../5原理/">
-
-
- 第5章 原理
-
- </a>
-
-
- <ul class="articles">
-
- <li class="chapter " data-level="1.7.1" data-path="../5原理/5-1工作原理概括.html">
-
- <a href="../5原理/5-1工作原理概括.html">
-
-
- 5-1 工作原理概括
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.7.2" data-path="../5原理/5-2输出文件分析.html">
-
- <a href="../5原理/5-2输出文件分析.html">
-
-
- 5-2 输出文件分析
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.7.3" data-path="../5原理/5-3编写Loader.html">
-
- <a href="../5原理/5-3编写Loader.html">
-
-
- 5-3 编写 Loader
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.7.4" data-path="../5原理/5-4编写Plugin.html">
-
- <a href="../5原理/5-4编写Plugin.html">
-
-
- 5-4 编写 Plugin
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.7.5" data-path="../5原理/5-5调试Webpack.html">
-
- <a href="../5原理/5-5调试Webpack.html">
-
-
- 5-5 调试 Webpack
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.7.6" data-path="../5原理/5-6原理总结.html">
-
- <a href="../5原理/5-6原理总结.html">
-
-
- 5-6 原理总结
-
- </a>
-
-
- </li>
- </ul>
-
- </li>
- <li class="chapter " data-level="1.8" >
-
- <span>
-
-
- 附录
-
- </span>
-
-
- <ul class="articles">
-
- <li class="chapter " data-level="1.8.1" data-path="../附录/常用Loaders.html">
-
- <a href="../附录/常用Loaders.html">
-
-
- 常用 Loaders
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.8.2" data-path="../附录/常用Plugins.html">
-
- <a href="../附录/常用Plugins.html">
-
-
- 常用 Plugins
-
- </a>
-
-
- </li>
- <li class="chapter " data-level="1.8.3" data-path="../附录/其它Webpack学习资源.html">
-
- <a href="../附录/其它Webpack学习资源.html">
-
-
- 其它 Webpack 学习资源
-
- </a>
-
-
- </li>
- </ul>
-
- </li>
-
- <li class="divider"></li>
- <li id="codefund_ad">
- <script src="https://codesponsor.io/scripts/5da524c5-301a-4ef4-8b42-696edd22e693/embed.js"></script>
- </li>
- </ul>
- </nav>
-
-
- </div>
- <div class="book-body">
-
- <div class="body-inner">
-
-
- <div class="book-header" role="navigation">
-
- <!-- Title -->
- <h1>
- <i class="fa fa-circle-o-notch fa-spin"></i>
- <a href=".." >4-10 使用 Tree Shaking</a>
- </h1>
- </div>
- <div class="page-wrapper" tabindex="-1" role="main">
- <div class="page-inner">
-
- <div id="book-search-results">
- <div class="search-noresults">
-
- <section class="normal markdown-section">
-
- <h1 id="4-10-使用-tree-shaking">4-10 使用 Tree Shaking</h1>
- <h2 id="认识-tree-shaking">认识 Tree Shaking</h2>
- <p>Tree Shaking 可以用来剔除 JavaScript 中用不上的死代码。它依赖静态的 ES6 模块化语法,例如通过 <code>import</code> 和 <code>export</code> 导入导出。
- Tree Shaking 最先在 Rollup 中出现,Webpack 在 2.0 版本中将其引入。</p>
- <p>为了更直观的理解它,来看一个具体的例子。假如有一个文件 <code>util.js</code> 里存放了很多工具函数和常量,在 <code>main.js</code> 中会导入和使用 <code>util.js</code>,代码如下:</p>
- <p><code>util.js</code> 源码:</p>
- <pre><code class="lang-js"><span class="hljs-keyword">export</span> <span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">funcA</span>(<span class="hljs-params"></span>) </span>{
- }
- <span class="hljs-keyword">export</span> <span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">funB</span>(<span class="hljs-params"></span>) </span>{
- }
- <span class="hljs-keyword">export</span> <span class="hljs-keyword">const</span> a = <span class="hljs-string">'a'</span>;
- </code></pre>
- <p><code>main.js</code> 源码:</p>
- <pre><code class="lang-js"><span class="hljs-keyword">import</span> {funcA} <span class="hljs-keyword">from</span> <span class="hljs-string">'./util.js'</span>;
- funcA();
- </code></pre>
- <p>Tree Shaking 后的 <code>util.js</code>:</p>
- <pre><code class="lang-js"><span class="hljs-keyword">export</span> <span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">funcA</span>(<span class="hljs-params"></span>) </span>{
- }
- </code></pre>
- <p>由于只用到了 <code>util.js</code> 中的 <code>funcA</code>,所以剩下的都被 Tree Shaking 当作死代码给剔除了。</p>
- <p>需要注意的是要让 Tree Shaking 正常工作的前提是交给 Webpack 的 JavaScript 代码必须是采用 ES6 模块化语法的,
- 因为 ES6 模块化语法是静态的(导入导出语句中的路径必须是静态的字符串,而且不能放入其它代码块中),这让 Webpack 可以简单的分析出哪些 <code>export</code> 的被 <code>import</code> 过了。
- 如果你采用 ES5 中的模块化,例如 <code>module.export={...}</code>、<code>require(x+y)</code>、<code>if(x){require('./util')}</code>,Webpack 无法分析出哪些代码可以剔除。</p>
- <blockquote>
- <p>目前的 Tree Shaking 还有些的局限性,经实验发现:</p>
- <ol>
- <li>不会对entry入口文件做 Tree Shaking。</li>
- <li>不会对<a href="4-12按需加载.html">异步分割</a>出去的代码做 Tree Shaking。</li>
- </ol>
- </blockquote>
- <h2 id="接入-tree-shaking">接入 Tree Shaking</h2>
- <p>上面讲了 Tree Shaking 是做什么的,接下来一步步教你如何配置 Webpack 让 Tree Shaking 生效。</p>
- <p>首先,为了把采用 ES6 模块化的代码交给 Webpack,需要配置 Babel 让其保留 ES6 模块化语句,修改 <code>.babelrc</code> 文件为如下:</p>
- <pre><code class="lang-json">{
- <span class="hljs-string">"presets"</span>: [
- [
- <span class="hljs-string">"env"</span>,
- {
- <span class="hljs-string">"modules"</span>: <span class="hljs-literal">false</span>
- }
- ]
- ]
- }
- </code></pre>
- <p>其中 <code>"modules": false</code> 的含义是关闭 Babel 的模块转换功能,保留原本的 ES6 模块化语法。</p>
- <p>配置好 Babel 后,重新运行 Webpack,在启动 Webpack 时带上 <code>--display-used-exports</code> 参数,以方便追踪 Tree Shaking 的工作,
- 这时你会发现在控制台中输出了如下的日志:</p>
- <pre><code>> webpack --display-used-exports
- bundle.js 3.5 kB 0 [emitted] main
- [0] ./main.js 41 bytes {0} [built]
- [1] ./util.js 511 bytes {0} [built]
- [only some exports used: funcA]
- </code></pre><p>其中 <code>[only some exports used: funcA]</code> 提示了 <code>util.js</code> 只导出了用到的 <code>funcA</code>,说明 Webpack 确实正确的分析出了如何剔除死代码。</p>
- <p>但当你打开 Webpack 输出的 <code>bundle.js</code> 文件看下时,你会发现用不上的代码还在里面,如下:</p>
- <pre><code class="lang-js"><span class="hljs-comment">/* harmony export (immutable) */</span>
- __webpack_exports__[<span class="hljs-string">"a"</span>] = funcA;
- <span class="hljs-comment">/* unused harmony export funB */</span>
- <span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">funcA</span>(<span class="hljs-params"></span>) </span>{
- <span class="hljs-built_in">console</span>.log(<span class="hljs-string">'funcA'</span>);
- }
- <span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">funB</span>(<span class="hljs-params"></span>) </span>{
- <span class="hljs-built_in">console</span>.log(<span class="hljs-string">'funcB'</span>);
- }
- </code></pre>
- <p>Webpack 只是指出了哪些函数用上了哪些没用上,要剔除用不上的代码还得经过 UglifyJS 去处理一遍。
- 要接入 UglifyJS 也很简单,不仅可以通过<a href="4-8压缩代码.html">4-8压缩代码</a>中介绍的加入 UglifyJSPlugin 去实现,
- 也可以简单的通过在启动 Webpack 时带上 <code>--optimize-minimize</code> 参数,为了快速验证 Tree Shaking 我们采用较简单的后者来实验下。</p>
- <p>通过 <code>webpack --display-used-exports --optimize-minimize</code> 重启 Webpack 后,打开新输出的 <code>bundle.js</code>,内容如下:</p>
- <pre><code class="lang-js"><span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">r</span>(<span class="hljs-params"></span>) </span>{
- <span class="hljs-built_in">console</span>.log(<span class="hljs-string">"funcA"</span>)
- }
- t.a = r
- </code></pre>
- <p>可以看出 Tree Shaking 确实做到了,用不上的代码都被剔除了。</p>
- <p>当你的项目使用了大量第三方库时,你会发现 Tree Shaking 似乎不生效了,原因是大部分 Npm 中的代码都是采用的 CommonJS 语法,
- 这导致 Tree Shaking 无法正常工作而降级处理。
- 但幸运的时有些库考虑到了这点,这些库在发布到 Npm 上时会同时提供两份代码,一份采用 CommonJS 模块化语法,一份采用 ES6 模块化语法。
- 并且在 <code>package.json</code> 文件中分别指出这两份代码的入口。</p>
- <p>以 <code>redux</code> 库为例,其发布到 Npm 上的目录结构为:</p>
- <pre><code>node_modules/redux
- |-- es
- | |-- index.js # 采用 ES6 模块化语法
- |-- lib
- | |-- index.js # 采用 ES5 模块化语法
- |-- package.json
- </code></pre><p><code>package.json</code> 文件中有两个字段:</p>
- <pre><code class="lang-json">{
- <span class="hljs-string">"main"</span>: <span class="hljs-string">"lib/index.js"</span>, <span class="hljs-comment">// 指明采用 CommonJS 模块化的代码入口</span>
- <span class="hljs-string">"jsnext:main"</span>: <span class="hljs-string">"es/index.js"</span> <span class="hljs-comment">// 指明采用 ES6 模块化的代码入口</span>
- }
- </code></pre>
- <p>在<a href="../2配置/2-4Resolve.html#mainFields">2-4Resolve mainFields</a> 中曾介绍过 <code>mainFields</code> 用于配置采用哪个字段作为模块的入口描述。
- 为了让 Tree Shaking 对 <code>redux</code> 生效,需要配置 Webpack 的文件寻找规则为如下:</p>
- <pre><code class="lang-js"><span class="hljs-built_in">module</span>.exports = {
- resolve: {
- <span class="hljs-comment">// 针对 Npm 中的第三方模块优先采用 jsnext:main 中指向的 ES6 模块化语法的文件</span>
- mainFields: [<span class="hljs-string">'jsnext:main'</span>, <span class="hljs-string">'browser'</span>, <span class="hljs-string">'main'</span>]
- },
- };
- </code></pre>
- <p>以上配置的含义是优先使用 <code>jsnext:main</code> 作为入口,如果不存在 <code>jsnext:main</code> 就采用 <code>browser</code> 或者 <code>main</code> 作为入口。
- 虽然并不是每个 Npm 中的第三方模块都会提供 ES6 模块化语法的代码,但对于提供了的不能放过,能优化的就优化。</p>
- <p>目前越来越多的 Npm 中的第三方模块考虑到了 Tree Shaking,并对其提供了支持。
- 采用 <code>jsnext:main</code> 作为 ES6 模块化代码的入口是社区的一个约定,假如将来你要发布一个库到 Npm 时,希望你能支持 Tree Shaking,
- 以让 Tree Shaking 发挥更大的优化效果,让更多的人为此受益。</p>
- <blockquote>
- <p>本实例<a href="http://webpack.wuhaolin.cn/4-10使用TreeShaking.zip" target="_blank">提供项目完整代码</a></p>
- </blockquote>
-
- </section>
-
- </div>
- <div class="search-results">
- <div class="has-results">
-
- <h1 class="search-results-title"><span class='search-results-count'></span> results matching "<span class='search-query'></span>"</h1>
- <ul class="search-results-list"></ul>
-
- </div>
- <div class="no-results">
-
- <h1 class="search-results-title">No results matching "<span class='search-query'></span>"</h1>
-
- </div>
- </div>
- </div>
- </div>
- </div>
-
- </div>
-
-
- <a href="4-9CDN加速.html" class="navigation navigation-prev " aria-label="Previous page: 4-9 CDN 加速">
- <i class="fa fa-angle-left"></i>
- </a>
-
-
- <a href="4-11提取公共代码.html" class="navigation navigation-next " aria-label="Next page: 4-11 提取公共代码">
- <i class="fa fa-angle-right"></i>
- </a>
-
-
-
- </div>
- <script>
- var gitbook = gitbook || [];
- gitbook.push(function() {
- gitbook.page.hasChanged({"page":{"title":"4-10 使用 Tree Shaking","level":"1.6.10","depth":2,"next":{"title":"4-11 提取公共代码","level":"1.6.11","depth":2,"path":"4优化/4-11提取公共代码.md","ref":"4优化/4-11提取公共代码.md","articles":[]},"previous":{"title":"4-9 CDN 加速","level":"1.6.9","depth":2,"path":"4优化/4-9CDN加速.md","ref":"4优化/4-9CDN加速.md","articles":[]},"dir":"ltr"},"config":{"plugins":["github-buttons","ga","modal","adsense"],"root":"./docs","styles":{"website":"styles/website.css","pdf":"styles/pdf.css","epub":"styles/epub.css","mobi":"styles/mobi.css","ebook":"styles/ebook.css","print":"styles/print.css"},"pluginsConfig":{"search":{},"adsense":{"client":"ca-pub-9697944574373240","slot":"9319793387","format":"auto","element":".page-inner section","position":"top"},"lunr":{"maxIndexSize":1000000,"ignoreSpecialCharacters":false},"modal":{"closable":true,"excludeUrls":["\\?q=.+$","index.html","前言.html","\\/$","1-.+","2-2Output","3-1使用ES6语言","3-2使用TypeScript语言","3-4使用SCSS语言.html","3-5使用PostCSS.html","3-6使用React框架","3-7使用Vue框架","3-8使用Angular2框架","3-9为单页应用生成HTML","3-10管理多个单页应用","3-17通过Node.jsAPI启动Webpack","3-20加载SVG"],"html":"<div style='text-align: center'><a href='http://union-click.jd.com/jdc?d=1AMcaz'><img src='http://p0.meituan.net/scarlett/df16c51ffb95186df6f75d8c0e22b965842464.png' width='306px' height='411px'><a/> <h2><a target='_blank' href='http://union-click.jd.com/jdc?d=1AMcaz'>购买本书纸质版</a></h2></div>"},"fontsettings":{"theme":"white","family":"sans","size":2},"highlight":{},"github-buttons":{"buttons":[{"user":"gwuhaolin","type":"follow","size":"small","width":"150"},{"user":"gwuhaolin","repo":"dive-into-webpack","type":"star","size":"small"}]},"ga":{"configuration":"auto","token":"UA-54964850-3"},"sharing":{"facebook":false,"twitter":false,"google":false,"weibo":true,"instapaper":false,"vk":false,"all":["facebook","google","twitter","weibo","instapaper"]},"theme-default":{"styles":{"website":"styles/website.css","pdf":"styles/pdf.css","epub":"styles/epub.css","mobi":"styles/mobi.css","ebook":"styles/ebook.css","print":"styles/print.css"},"showLevel":false}},"theme":"default","author":"吴浩麟","pdf":{"pageNumbers":true,"fontSize":12,"fontFamily":"Arial","paperSize":"a4","chapterMark":"pagebreak","pageBreaksBefore":"/","margin":{"right":62,"left":62,"top":56,"bottom":56}},"structure":{"langs":"LANGS.md","readme":"README.md","glossary":"GLOSSARY.md","summary":"SUMMARY.md"},"isbn":"9787121331725","variables":{},"title":"深入浅出 Webpack","language":"zh-hans","gitbook":"*","description":"本书按照入门、配置、实战、优化、原理的路线层层深入,涵盖了 Webpack 的方方面面。对于小白,本书可以作为 Webpack 入门教程,对于老手,本书可以进一步加深你对 Webpack 的理解。"},"file":{"path":"4优化/4-10使用TreeShaking.md","mtime":"2018-12-17T02:01:55.796Z","type":"markdown"},"gitbook":{"version":"3.2.3","time":"2019-03-17T07:14:49.167Z"},"basePath":"..","book":{"language":""}});
- });
- </script>
- </div>
-
- <script src="../gitbook/gitbook.js"></script>
- <script src="../gitbook/theme.js"></script>
-
-
- <script src="../gitbook/gitbook-plugin-github-buttons/plugin.js"></script>
-
-
-
- <script src="../gitbook/gitbook-plugin-ga/plugin.js"></script>
-
-
-
- <script src="../gitbook/gitbook-plugin-modal/index.js"></script>
-
-
-
- <script src="../gitbook/gitbook-plugin-adsense/plugin.js"></script>
-
-
-
- <script src="../gitbook/gitbook-plugin-search/search-engine.js"></script>
-
-
-
- <script src="../gitbook/gitbook-plugin-search/search.js"></script>
-
-
-
- <script src="../gitbook/gitbook-plugin-lunr/lunr.min.js"></script>
-
-
-
- <script src="../gitbook/gitbook-plugin-lunr/search-lunr.js"></script>
-
-
-
- <script src="../gitbook/gitbook-plugin-sharing/buttons.js"></script>
-
-
-
- <script src="../gitbook/gitbook-plugin-fontsettings/fontsettings.js"></script>
-
-
- </body>
- </html>
|